The present invention relates to an acoustic fluid machine based on pressure variation by acoustic resonance and especially to an acoustic fluid machine suitable for use as an air compressor, a cooling compressor and a vacuum pump.
Recently acoustic compressors have attracted considerable attention, the compressors being grounded on pressure variation of large amplitude standing acoustic waves generated by resonance in acoustic resonators.
An acoustic resonator that is important in an acoustic fluid machine such as an acoustic compressor comprises a linear pipe having an internal constant cross-sectional area in EP 0 447 134 A2, and a conical pipe in which an internal cross-sectional area varies in U.S. Pat. No. 5,319,938 A and EP 0 570 177 A2.
When a linear pipe is used as acoustic resonator, waveform becomes steeper owing to nonlinearity with increase in amplitude to generate propagating shock waves in the acoustic resonator. Thus, increase rate of pressure amplitude in the acoustic resonator with respect to amplitude increase in a driving source decreases rapidly to cause acoustic saturation.
When a conical pipe is used as acoustic resonator, shock waves are suppressed, and larger pressure variation amplitude in the acoustic resonator is obtained in proportion to input amplitude increase of the driving source.
However, it is difficult to obtain industrially applicable pressure ratio in the linear or conical pipe, and resonance area is variable with variation in acceleration of the driving sound source depending on temperature change. Specifically, resonance points are likely to be shifted, so that it is difficult to keep resonance points, which results in difficulty in obtaining a stable acoustic compressor.